This invention relates to a steady operation method for a variable hydraulic machine or machinery under the steadily operating condition capable of controlling or limiting the vibration of the hydraulic machine or the increasing of a pressure variation due to the variation of the revolution speed in a hydroelectric power plant operated at a variable speed.
Recently, variable operating systems have been developed and utilized as systems for operating the hydraulic machine with the highest efficiency such as water turbines and pump turbines with large head variable and load variable (for example, refer to Japanese Patent Laid-Open Publication Nos. 57-113971, 58-18595 and 58-222981).
The outline of the variable operating system of the type described above will be explained briefly hereunder in conjunction with FIGS. 1 through 3.
FIG. 1 is a graph having the abscissa representing the revolution speed n.sub.1 per a unit head and the ordinate representing the flow rate q.sub.1 per a unit head and the graph shows the constant efficiency curve 21 and the maximum efficiency curve 22, wherein character n.sub.1 equals to N/.sqroot.H, q.sub.1 =Q/.sqroot.H and p=.alpha..eta.QH, and letter N designates revolution speed, Q: flow rate, H: head, p: output, .alpha.: constant, and .eta.: efficiency.
As will be understood from FIG. 1, under the conditions of H=.sqroot.H.sub.0 and Q=Q.sub.0, i.e. q.sub.1 =q.sub.0 (=Q.sub.0 /H.sub.0), the maximum efficiency can be obtained at only the point A on which the revolution speed n.sub.0 is designated by n.sub.0 =N.sub.0 /.sqroot.H.sub.0.
FIG. 2 is a graph representing the relationship between the flow rate Q and the efficiency .eta. with respect to the water turbine output p in a case where the revolution speed n.sub.1 is constant per the unit head, i.e. n.sub.1 =n.sub.0 and the maximum efficiency is obtained by only the point B, which corresponds to the point A of FIG. 1.
In FIG. 1, a broken line 22 shows the maximum efficiency curve combining the loci of the maximum efficiency points with respect to the flow rate q.sub.1 and the revolution speed n.sub.1 to the unit head, and the maximum efficiency curve corresponding to that of FIG. 2 is shown by a broken line 32 in FIG. 2.
FIG. 3 is a block diagram of one example of a variable speed operation system for controlling the revolution speed so as to always achieve the most effective operation in accordance with the head or the load. In the system illustrated in FIG. 3, the head H and the load L are detected by a head detecting device 1 and a load detecting device 2, respectively, and signals representing the detected head and load are then transmitted from these devices 1 and 2 into a signal converter 3 thereby to calculate the most available revolution speed N, which is converted into a revolution speed signal which is thereafter transmitted into a revolution speed controller 4. According to this manner, a generator or generator-motor 5 and a hydraulic machine directly connected thereto can be operated at the most available revolution speed N.
In the conventional technique described hereinbefore, the most available revolution speed N in the variable speed operation of the hydraulic machine 6 is merely set in accordance with the head H or the load L based on the performance condition of the hydraulic machine 6, and in other words, the revolution speed N is merely outputted as a revolution speed signal from the signal converter 3 or the like.
In general, however, in a variable hydraulic machine, the revolution speed of the hydraulic machine is varied, so that the hydraulically vibrating frequency applied by hydraulic influence due to a runner of the machine to a stationary portion, not shown, such as an upper cover and the hydraulically vibrating frequency applied by hydraulic buffer from a guide vane on the stationary side to the runner are varied in accordance with the variation of the revolution speed.
Accordingly, during the process of changing the revolution speed, in a case where the hydraulically vibrating frequency accords with the natural frequency of the hydraulic machine, an excessive abnormal vibration will be generated by the resonance phenomenon, which may result in the unoperable condition of the hydraulic machine.
On the other hand, with a hydraulic machine operating at a constant revolution speed, the resonance phenomenon mentioned above can be obviated by designing the hydraulic machine so that the natural frequency of the machine is not in accord with the hydraulically vibrating frequency. With the hydraulic machine operating at the variable revolution speed for the high performance operation, however, the resonance phenomenon will occur when the hydraulically vibrating frequency is overlaped, even though temporarily, with the natural frequency, and thus, the problem of the resonance phenomenon cannot be obviated and constitutes a matter to be solved.